Wind turbine rotor blade with vortex generators

ABSTRACT

A rotor blade of a wind turbine including at least one vortex generator is provided. The vortex generator is attached to the surface of the rotor blade and is located at least partially within the boundary layer of the airflow flowing across the rotor blade. The vortex generator is exposed to a stagnation pressure, which is caused by the fraction of the airflow passing over the vortex generator and of which the magnitude depends on the velocity of the fraction of the airflow passing over the vortex generator. The vortex generator is arranged and prepared to change its configuration depending on the magnitude of the stagnation pressure acting on the vortex generator. Furthermore, an aspect relates to a wind turbine for generating electricity with at least one such rotor blade.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European application No. EP 16178023having a filing date of Jul. 5, 2016, the entire contents of which arehereby incorporated by reference.

FIELD OF TECHNOLOGY

The following relates to a rotor blade of a wind turbine comprising atleast one vortex generator. Furthermore, embodiments of the inventionrelates to a wind turbine for generating electricity comprising at leastone such rotor blade.

BACKGROUND

Vortex generators are well known devices for manipulating the airflowflowing across the surface of a rotor blade of a wind turbine. Thefunction of a vortex generator is to generate vortices downstream of thearea where the vortex generator is mounted to the surface of the rotorblade. The generated vortices may re-energize the boundary layer closeto the surface of the rotor blade. This re-energization of the boundarylayer may delay or prevent stall. The delay or elimination of stall atthe section of the rotor blade where the vortex generators are mountedgenerally increases the lift coefficient of the rotor blade at thissection. The increase of the lift is generally desirable. An increase ofthe lift generally correlates with an increase of the load of the rotorblade. This increase of the load of the rotor blade may be undesirable.

Therefore, there exists the desire to provide a concept how toselectively activate or deactivate, respectively, a vortex generator fora rotor blade of a wind turbine.

SUMMARY

According to embodiments of the invention there is provided a rotorblade of a wind turbine comprising at least one vortex generator,wherein the vortex generator is attached to the surface of the rotorblade. The vortex generator is located at least partially within theboundary layer of the airflow flowing across the rotor blade. The vortexgenerator is exposed to a stagnation pressure, which is caused by thefraction of the airflow passing over the vortex generator and of whichthe magnitude depends on the velocity of the fraction of the airflowpassing over the vortex generator. Furthermore, the vortex generator isarranged and prepared to change its configuration depending on themagnitude of the stagnation pressure acting on the vortex generator,such that, with increasing stagnation pressure in the boundary layer,the ability of the vortex generator to generate vortices decreases.

The boundary layer is the layer of the airflow in the immediate vicinityof the surface of the rotor blade. The boundary layer is also referredto as “surface boundary layer”.

In the boundary layer the effects of viscosity are significant. Thethickness of the boundary layer is defined as the distance of thesurface of the rotor blade at which the velocity of the airflow is 99%of the free stream velocity. For the surface of the rotor blade, theconfining, i.e. limiting surface of the rotor blade is taken, which istypically referred to as the suction side and the pressure side of therotor blade, respectively.

The airflow flowing across the rotor blade is understood to be thetypical airflow flowing from the leading edge section to the trailingedge section of the rotor blade. Depending on the particular set up andoperation mode of the wind turbine, and depending on the concretedirection of the impinging airflow, the direction of the airflow mayvary. However, in general, the airflow flowing across the rotor blade issubstantially parallel to the chordwise direction of the rotor blade.

The expression “the airflow passes over the vortex generator” can inother words be described as “the airflow impinges on the vortexgenerator”.

The stagnation pressure, which is sometimes also referred to as thePitot pressure, is defined as the pressure built up in a fluid which isbrought to rest isentropically. For a low velocity flow which can beassumed to be incompressible, the stagnation pressure is equal to thesum of a static pressure and a dynamic pressure. The static pressure isto be understood as being the free stream hydrostatic pressure, whichfor example for an open flow application such as a wind turbine isequivalent with the ambient pressure. The term dynamic pressure refersto the kinetic energy of a fluid per unit mass, and is thereforedependent on the velocity of the airflow. If a fluid is brought to restisentropically, all the kinetic energy per unit mass of the fluid isconverted into pressure, and therefore the pressure at a stagnationpoint (a point where the fluid is at rest) equals the sum of the staticand the dynamic pressure. The stagnation pressure can be measured usinga so-called Pitot tube.

A key aspect of embodiments of the present invention is that the varyingmagnitude of the stagnation pressure is used in order to activate ordeactivate selectively the vortex generator of the turbine blade. As thestatic pressure is assumed to be substantially equal during the relevantoperation conditions of the wind turbine, it is actually the variationof the dynamic pressure which causes the activation or deactivation ofthe vortex generator.

The dynamic pressure may increase or decrease due to the increase ordecrease of the thickness of the boundary layer. A thin boundary layer,for example, leads to a high dynamic pressure, while a thick boundarylayer involves a small dynamic pressure. Therefore, in other words, theinventive concepts can be described as well by a selective activation ordeactivation of the vortex generator depending on the thickness of theboundary layer.

Advantageously, the activation or deactivation of the vortex generatoroccurs passively. Thus, an actively driven actuator is not necessary inorder to activate the vortex generator. Instead, by the pure increase ofthe stagnation pressure, a change in the configuration of the vortexgenerator is caused.

For this purpose, in a particular embodiment of the invention, the rotorblade comprises a pressure tube extending substantially upstream fromthe vortex generator for guiding a fraction of the airflow flowingacross the rotor blade to an inflatable element.

Depending on the thickness of the boundary layer, a high velocityairstream or a low velocity airstream flows through the pressure tubeand impinges, i.e. hits or enters, the inflatable element. If thedynamic pressure, and consequently also the stagnation pressure, issmall, the inflatable element is not inflated or only little inflated.In contrast, for high dynamic and stagnation pressures, a high velocityairflow is flowing through the pressure tube, leading to the inflatableelement to be inflated to a significant extent.

In an alternative embodiment, the varying stagnation pressure is onlyused as a trigger for triggering an actuator for activating the vortexgenerator. This actuator may e.g. be electrically or hydraulicallydriven. As an example, the actuator may inflate or deflate theinflatable element being associated with the vortex generator.

Note that the ability of the vortex generator to generate vorticesdecreases with increasing stagnation pressure in the boundary layer.

In other words, the rotor blade is designed such that a thin boundarylayer leads to a high stagnation pressure and to a deactivation of thevortex generator, while a thick boundary layer leads to a smallstagnation pressure, resulting in an activation of the vortex generator.

It is known to place vortex generators almost at any spanwise positionof the rotor blade. Therefore, the skilled person has to make a choicewhere to beneficially place the inventive vortex generator on the rotorblade. It is suggested to place and situate the vortex generator in theoutboard half, in particular in the outboard third, of the rotor blade.

This choice is preferred because here the impact on the liftcoefficient, and hence, on the load of the rotor blade is particularlystrong. With the notion “outboard”, the area adjacent to the tip of therotor blade is meant.

Examples of an inflatable element which is suitable to be used in thecontext of embodiments of this invention, a hose or a pressure chamberare to be mentioned.

A hose has the advantage that it can be designed separately from therest of the rotor blade and may also be exchanged easily after a certaintime of operation.

On the other hand, a pressure chamber, which has to be understood as acavity running substantially in spanwise direction, is fully integratedin the profile of the rotor blade. No additional components and partsare used, which is advantageous. However, it is difficult to retrofit,for example, a rotor blade by a pressure chamber.

In another embodiment of the invention, the vortex generator is at leastpartially embedded into the surface of the rotor blade.

In this case, a vortex generator is seen as being active or activatedwhen the vortex generator is sticking out from the surface. In otherwords, it is projecting away from the surface of the rotor blade. Incontrast to this scenario, a vortex generator is seen as deactivatedleading to a decrease in lift and load, if the vortex generator isentirely or at least partially embedded into the surface of the rotorblade.

Note that the notion “into the surface of the rotor blade” signifies thepresence of a recess or a groove or similar design options. As theremaining surface portions of the general surface of the rotor blade isunchanged, a vortex generator is called to be “embedded into thesurface” if it somehow intersects the expected contour of the rotorblade.

A possibility to selectively embed the vortex generator is that in thiscase not only the ability to generate vortices is prevented or reducedbut also the drag of the rotor blade. This is desirable because areduced drag normally leads to an increase of the performance of therotor blade and, hence, of the wind turbine. As mentioned above, it ispreferred that the portion of the vortex generator which is embeddedinto the surface of the rotor blade increases with increasing stagnationpressure. This ultimately leads to a selective deactivation of thevortex generator with increasing stagnation pressure.

In another embodiment of the invention, the vortex generator is able tobend depending on the value of the stagnation pressure acting on thevortex generator.

In this embodiment, an inflatable element is not necessarily needed.Instead, by the mere presence and design of the vortex generator theconfiguration, in particular the shape of the vortex generator can bevaried. This may be realized by an elastic portion at the vortexgenerator.

Again, in a preferred configuration, at high velocities of the airflow,the vortex generator is bent down towards the surface of the rotor bladeand is consequently at least partially deactivated. This has theconsequence that the ability to generate vortices is reduced.

If, on the other hand, the boundary layer thickness is large, thus, thestagnation pressure is small, the vortex generator moves away from thesurface. Thus, a greater portion of the vortex generator is stickingout. In other words, the vortex generator projects away from the surfacesuch that the ability of the vortex generator to generate vortices isincreased.

A similar, but slightly different, design is a vortex generator which isable to straighten relative to a direction of the airflow depending onthe value of the stagnation pressure acting on the vortex generator.

In this case, it is preferred that a pair of vortex generators exists atleast at the surface of the rotor blade. Then, for example, at lowvelocity of the airflow (thick boundary layer), a large angle is presentbetween the two vortex generators of the pair, while at high velocity ofthe airflow (thin boundary layer), the angle between the two vortexgenerators of the pair is small, even leading to a substantiallyparallel configuration of the two vortex generators of the pair.

In practice, this can also be realized by an elastic portion of thevortex generator.

Finally, embodiments of the present invention also relate to a windturbine for generating electricity comprising at least one rotor bladeaccording to one of the embodiments described above.

BRIEF DESCRIPTION

Some of the embodiments will be described in detail, with reference tothe following figures, wherein like designations denote like members,wherein:

FIG. 1 shows a rotor blade of a wind turbine;

FIG. 2 shows a cross sectional view of the rotor blade at a certainspanwise position;

FIG. 3 shows a perspective view of a first embodiment of vortexgenerators;

FIG. 4 shows a cut-away view of a first embodiment of a thick embodimentof a vortex generators;

FIG. 5 shows a cut-away view of a first embodiment of a thin embodimentof a vortex generators;

FIG. 6 shows a side view of a second embodiment of vortex generatorshaving a deflated pressure chamber;

FIG. 7 shows a side view of a second embodiment of vortex generatorshaving an inflated pressure chamber;

FIG. 8 shows a third embodiment of vortex generators with a slightlyinflated pressure chamber;

FIG. 9 shows a third embodiment of vortex generators with an inflatedpressure chamber;

FIG. 10 shows a fourth embodiment of vortex generators;

FIG. 11 shows a fourth embodiment of vortex generators with a thickboundary layer;

FIG. 12 shows a fourth embodiment of vortex generators with a thinboundary layer;

FIG. 13 shows a fifth embodiment of vortex generators; and

FIG. 14 shows a fifth embodiment of vortex generators wherein adjacentvortex generators comprise a significant angle relative to each other.

Note that the following drawings are only schematically. Similar oridentical reference signs are used throughout the drawings.

DETAILED DESCRIPTION

FIG. 1 shows a rotor blade 20 of a wind turbine. The rotor blade 20comprises a root 21 and a tip 22. The root 21 and the tip 22 areconnected by a virtual line, which is referred to as the span 25. Thespan 25 can be described as a virtual line, which is a straight line andwhich not necessarily exactly connects the root 21 and the tip 22. Thiswould be the case if the rotor blade was a straight rotor blade. If,however, as for example illustrated in the example of the rotor blade ofFIG. 1, the rotor blade is a slightly swept rotor blade, the tip may beslightly separate from the span 25. If the rotor blade is designed for apitchable wind turbine, the span 25 can be associated and coincides withthe pitch axis of the rotor blade.

Another characteristic feature and parameter of rotor blades of a windturbine are the chords of the rotor blade. The chords 26, which are alsoreferred to as the chord lines, can be defined and assigned for everyspanwise position from the root to the tip of the rotor blade. The chord26 is defined as the straight line being perpendicular to the span 25and connecting the leading edge 23 of the rotor blade 20 with thetrailing edge 24 of the rotor blade 20.

A particular chord length can be assigned to each chord 26. The maximumchord 261 is understood to be that chord which has the maximum length.The portion of the rotor blade where the maximum chord 261 is present isreferred to as the shoulder 262 of the rotor blade. The part of therotor blade between the shoulder 262 and the tip 22 is also referred toas the airfoil portion of the rotor blade. On the other hand, the partof the rotor blade between the shoulder 262 and the root 21 is referredto a transition and root region of the rotor blade.

FIG. 2 shows a cross sectional view at a certain spanwise position ofthe airfoil portion of the rotor blade. Again, the leading edge 23 andthe trailing edge 24 can be seen. Additionally, the trailing edgesection 241 and the leading edge section 231 are referenced in FIG. 2.The leading edge section 231 is defined as that section surrounding theleading edge 23 reaching from the leading edge 23 to a chordwiseposition of ten percent of the chord length as measured from the leadingedge 23. Likewise, the trailing edge section 241 of the rotor blade isdefined as that section of the rotor blade which extends between ninetypercent chordwise position as measured from the leading edge 23 untilthe very trailing edge 24.

FIG. 2 also illustrate the airflow 40 flowing from the leading edgesection 231 to the trailing edge section 241 of the rotor blade. As canbe seen, the airflow 40 is subdivided into a suction side airflow 41 anda pressure side airflow 42. The separation of the airflow occurs at thestagnation point 29. Typically, the stagnation point 29 is located atthe pressure side 28 of the rotor blade, but may also be located at thesuction side 27 of the rotor blade. The exact position of the stagnationpoint 29 depends on a variety of factors, mainly it depends on the angleof attack and the pitch movement of the rotor blade.

FIG. 3-5 show a first embodiment of the present invention. Inparticular, a first embodiment of a vortex generator 30 is disclosed,which can be used and which is a part of a first embodiment of aninventive rotor blade.

FIG. 3 shows a perspective view of four pairs of such vortex generators30. These vortex generators 30 are attached to a housing 35, which, as awhole, can be attached and mounted onto the surface, e.g. the suctionside surface, of the rotor blade. An important feature of thearrangement as illustrated in FIG. 3 is the pressure tube 31. Thepressure tube 31 consists of a relatively small diameter tube which isarranged upstream of the vortex generator. The arrangement furthermorecomprises an inflatable member, namely a hose 32. This hose 32 islocated within the housing 35. The hose 32 is able to push the vortexgenerator 30 downwards towards the surface of the rotor blade which isexemplarily referenced by the suction side 27. In order to facilitate orenable such a bending of the vortex generator, the vortex generator 30comprises an elastic portion 34.

FIG. 4 illustrates the scenario of a thick boundary layer—confer to theshown velocity profile 43 in FIG. 4. In contrast to FIG. 4, FIG. 5illustrates the scenario of a thin boundary layer—confer to the velocityprofile 43 as illustrated in FIG. 5. As it can be seen, depending on thethickness of the boundary layer, the hose 32 is inflated or not whichleads to an upwardly projecting vortex generator 30 or a vortexgenerator which is almost in contact with the suction side 27 of therotor blade.

Note that in the first embodiment of the invention, the housing 35 isdesigned as a relatively stiff and rigid element. This means that itsshape is substantially independent on the state of the hose 32. Whetherthe hose 32 is inflated (as in FIG. 5) or not (as in FIG. 4)—the housinghas the same cross-sectional profile. As a consequence, the airflow,which is passing over the housing 35 is not influenced by the factwhether the hose 32 is inflated or deflated.

FIGS. 6 and 7 shows a second embodiment of the invention. Here,inflatable element is exemplarily designed as a pressure chamber 33. Thepressure chamber may be in a deflated state (confer FIG. 6)—which is thecase for a thick boundary layer, i.e. for a low stagnation pressure—orit may be in an inflated state (confer FIG. 7)—which is the case for athin boundary layer, i.e. for a high stagnation pressure.

The pressure chamber 33 is accommodated and surrounded by a housing 35.In this embodiment, the housing is made of a flexible material. As aconsequence, and contrary to the first embodiment as illustrated inFIGS. 3 to 5, the housing does change its shape depending of the stateof the inflatable element.

Descriptively speaking, the housing 35 represents a “bump” for theairflow passing over it. Note that the airflow, which is passing overthe housing 33, is influenced by the fact whether the pressure chamber33 is inflated or deflated.

FIGS. 8 and 9 disclose a third embodiment of a vortex generator. Thistime, the vortex generator 30 is partially embedded into the surface,e.g. into the suction side 27 of the rotor blade. In other words, therotor blade is provided with a recess or groove at its suctions side 27.In this groove, a device or arrangement comprising a pressure chamber 33can be seen. This pressure chamber is connected with a pressure tube 31.Depending on the stagnation pressure which is guided through thepressure tube 31, the pressure chamber 33 is either inflated (conferFIG. 9) or it is not or only slightly inflated (confer FIG. 8). As aconsequence, the vortex generator 30 is either submerged and embeddedinto the surface of the rotor blade (confer FIG. 9) or it projects awayand sticks out of the surface (confer FIG. 8).

This third embodiment has the advantage that additional drag from theattachment portion as shown in the first embodiment as illustrated inFIGS. 3-5, is avoided. Thus, additional drag from the attachmentportion, but also from the vortex generator as such, is reduced.

FIGS. 10, 11 and 12 disclose a fourth embodiment of an inventive vortexgenerator. This time no inflatable element, such as a pressure chamberor a hose, is used. Instead, it is the direct and sole design andconfiguration of the vortex generator 30 which leads to a changingconfiguration of the vortex generator in dependence of the velocityprofile 43. See FIG. 11 for these scenarios of a thick boundary layer.As a consequence, the stagnation pressure at the position of the vortexgenerator 30 is small, thus the vortex generator which comprises anelastic portion 34 and which is bent upwards, i.e. away from the surfaceof the rotor blade, is projecting away and is able to generate vorticesto a considerable extent. In contrast to that, FIG. 12 shows thesescenarios of a thin boundary layer which can be seen by the velocityprofile 43 leading to the bending down of the vortex generator 30towards the suction side surface of the rotor blade. In theconfiguration as illustrated in FIG. 12 the ability of generatingvortices by the vortex generator is heavily reduced.

Finally, the FIGS. 13 and 14 disclose a fifth embodiment of theinvention. Similar to the fourth embodiment, no inflatable element orthe like is present. Instead, the vortex generators themselves againcomprise an elastic portion 34. This elastic portion 34 is designed suchthat for a thin boundary layer, as illustrated in FIG. 13, the vortexgenerators 30 are almost parallel to each other. They can also bedescribed as being straightened by the high stagnation pressure of theairflow impinging on the vortex generator. In contrast to FIG. 13, FIG.14 shows this scenario of a thick boundary layer, wherein the relativelysmall stagnation pressure is not able to overcome the pre-bent of theelastic portion 34 of the vortex generators 30. Thus, the adjacentvortex generators comprise a significant angle relative to each other.In this case, the ability to generate vortices is increased, compared tothe straightened scenario as illustrated in FIG. 13.

Although the present invention has been disclosed in the form ofpreferred embodiments and variations thereon, it will be understood thatnumerous additional modifications and variations could be made theretowithout departing from the scope of the invention.

For the sake of clarity, it is to be understood that the use of “a” or“an” throughout this application does not exclude a plurality, and“comprising” does not exclude other steps or elements.

The invention claimed is:
 1. A rotor blade of a wind turbine comprisingat least one vortex generator, wherein the at least one vortex generatoris attached to a surface of the rotor blade, the at least one vortexgenerator is located at least partially within a boundary layer of anairflow flowing across the rotor blade, the at least one vortexgenerator is exposed to a stagnation pressure, which is caused by afraction of the airflow passing over the at least one vortex generatorand of which a magnitude depends on a velocity of the fraction of theairflow passing over the at least one vortex generator, wherein aconfiguration of the at least one vortex generator changes depending onthe magnitude of the stagnation pressure acting on the at least onevortex generator and not depending upon an actively driven actuator,wherein when the magnitude of the stagnation pressure acting on thevortex generator increases, the stagnation pressure changes theconfiguration of the vortex generator to decrease generation ofvortices, and wherein when the magnitude of the stagnation pressure onthe vortex generator decreases, the stagnation pressure changes theconfiguration of the vortex generator to increase generation ofvortices.
 2. The rotor blade according to claim 1, wherein the at leastone vortex generator is situated in an outboard half of the rotor blade.3. The rotor blade according to claim 1, wherein the at least one vortexgenerator comprises an inflatable element, selected from a hose or apressure chamber.
 4. The rotor blade according to claim 3, wherein therotor blade comprises a pressure tube extending upstream from the atleast one vortex generator for guiding a portion of the fraction of theairflow flowing across the rotor blade to the inflatable element.
 5. Therotor blade according to claim 3, wherein the at least one vortexgenerator is at least partially embedded into the surface of the rotorblade.
 6. The rotor blade according to claim 1, wherein the at least onevortex generator is able to bend depending on a value of the stagnationpressure acting on the at least one vortex generator.
 7. The rotor bladeaccording to claim 6, wherein an elastic portion of the at least onevortex generator enables the at least one vortex generator to bend orstraighten.
 8. A wind turbine for generating electricity with at leastone rotor blade according to claim
 1. 9. The rotor blade according toclaim 1, wherein the at least one vortex generator is situated in anoutboard third of the rotor blade.
 10. A vortex generator, wherein thevortex generator is attached to a surface of a rotor blade, wherein thevortex generator is located on the surface of the rotor blade such thatthe vortex generator is located at least partially within a boundarylayer of an airflow flowing across the rotor blade, wherein the vortexgenerator is configured to be acted upon by a stagnation pressure causedby a fraction of the airflow passing over the vortex generator, thestagnation pressure having a magnitude dependent on a velocity of thefraction of the airflow passing over the vortex generator, wherein thestagnation pressure acting upon the vortex generator directly changes aconfiguration of the vortex generator without an actively drivenactuator, wherein when the magnitude of the stagnation pressure on thevortex generator increases, the stagnation pressure changes theconfiguration of the vortex generator to decrease generation ofvortices, and wherein when the magnitude of the stagnation pressure onthe vortex generator decreases, the stagnation pressure changes theconfiguration of the vortex generator to increase generation ofvortices.
 11. The vortex generator according to claim 10, wherein thestagnation pressure acting upon the vortex generator and directlychanging the configuration of the vortex generator passively activatesand deactivates the vortex generator in response to variations in thestagnation pressure.
 12. The vortex generator according to claim 10,wherein the vortex generator comprises an inflatable element, whereinthe inflatable element receives a portion of the fraction of the airflowflowing across the rotor blade through a pressure tube extendingupstream from the vortex generator.
 13. A rotor blade of a wind turbinecomprising: a vortex generator, wherein the vortex generator is attachedto a surface of the rotor blade, wherein the vortex generator is locatedon the surface of the rotor blade such that the vortex generator islocated at least partially within a boundary layer of an airflow flowingacross the rotor blade, wherein the vortex generator comprises aninflatable element, wherein the vortex generator is configured to beacted upon by a stagnation pressure caused by a fraction of the airflowpassing over the vortex generator, the stagnation pressure having amagnitude dependent on a velocity of the fraction of the airflow passingover the vortex generator, and wherein a configuration of the vortexgenerator is configured to change depending on the magnitude of thestagnation pressure acting upon the vortex generator, such that, withincreasing stagnation pressure in the boundary layer, the configurationof the vortex generator changes to decrease generation of vortices, andwith decreasing stagnation pressure in the boundary layer, theconfiguration of the vortex generator changes to increase generation ofvortices, and a pressure tube extending upstream from the vortexgenerator for guiding a portion of the fraction of the airflow flowingacross the rotor blade to the inflatable element.
 14. The rotor blade ofa wind turbine according to claim 13, wherein the configuration of thevortex generator changes configuration without an actively drivenactuator.